Polyolefin plastic film is used to construct a wide variety of packages such as bags and pouches that hold consumer goods. For example, bags holding stacks of disposable diapers or hygiene articles, pouches for wet wipes, and bags containing granular laundry detergent are often made from plastic film. The plastic film that forms a package may be a single layer of film (called a monofilm), or a combination of layers that for example can be co-extruded, a laminate of separately produced layers that are adhered to one another, or an extrusion lamination whereas one layer is extruded onto another previously formed layer(s).
The specific composition of the film or films that make up the package are selected for a variety of characteristics including liquid or gas permeability, appearance, and strength. Another relevant characteristic of plastic film used for packaging is opacity. The level of opacity of the plastic film used in a package impacts the appearance of the package by controlling the extent to which the package's contents are visible through the package. In some circumstances, a higher opacity film may be desirable to protect the contents from exposure to light. Additives such as titanium oxide or other white or colored pigments are mixed with the resin for the purpose of increasing the opacity of a film. In general, decreasing the amount of resin by in a film by making the film thinner will in turn reduce its opacity.
Many plastic film packages include opening features, such as, for example, lines of weakness and/or peelable labels covering die cut openings. These lines of weakness and/or peelable labels covering die cut openings are configured to provide convenient consumer access to the contents of the package while maintaining the integrity of the unopened package during shipment and storage. Lines of weakness, such as perforations or scores, provide a mechanism by which the consumer can, in a controlled manner, tear open a package along a predetermined opening trajectory. The label and die cut dispensing opening combination may be configured to provide a re-sealable package for items that require retention of moisture and/or other product ingredients within the package and/or items for which it is desirable to exclude contamination. The die cut defines the dispensing opening through which items are dispensed. The label is sized to overlap the perimeter of the die cut dispensing opening. The label tears the die cut from the package the first time the label is peeled from the package. The label may be capable of completely re-covering and re-sealing the dispensing opening formed by the die cut.
Much of the cost associated with plastic film packages is the cost of the plastic resin that is used to make the film. Because the amount of plastic resin in the film is directly related to the caliper (or thickness) of the film, efforts to reduce cost in plastic film packages typically involve using a lower caliper film that can still provide the necessary characteristics for a particular package. Because lower caliper film is typically weaker in terms of inherent film tear strength, changing to a lower caliper film in packages that includes an opening feature (e.g., lines of weakness or die cut dispensing openings) requires a redesign of the opening feature to compensate for the lower tear strength of the film. For example, the cuts in a line of perforations may be made shorter to leave more film intact between the cuts to resist unintentional tearing of the line of perforations. Scores in the film may be made more shallow to provide additional strength to resist unintentional tearing of a lower caliper film. Film connections between the cuts that define a die cut may be made longer to resist unintentional separation of the die cut from the film. The redesign of the opening feature is costly in terms of engineering and evaluation time. In addition, the redesign of the opening feature typically requires laborious adjustments of various manufacturing components and processes that create the opening feature on the film and possibly the purchase of new tooling as well.
Recent technological developments have made it feasible to produce foamed polyolefin film of suitable thickness (from about 10 microns to about 250 microns) and strength for the types of packages described above. Several exemplary foamed polyolefin films that are suitable for packages are described in European Patent No. 1 646 677. The use of foamed thin film allows for replacement of part of the resin (e.g., from about 5% to about 50% by weight) with gaseous bubbles that are formed or incorporated in the film during a foaming process. Because the voids or cells left by the bubbles occupy volume that was formerly filled with resin, foamed film allows for a reduction in resin without a corresponding reduction in film caliper. One notable feature of foamed thin films is that they have a rough surface texture as compared to a non-foamed film of substantially the same caliper.
In spite of the cost savings in terms of resin usage, foamed thin film has not been used for packages with opening features (e.g., lines of weakness or die cut dispensing openings with labels). This may be because it would be expected that the use of a foamed thin film of substantially the same caliper (but with significantly less resin) would necessitate redesign of the opening features to compensate for reduced tear strength. As discussed above, such a redesign of the opening features would require engineering and test time, labor intensive adjustments to manufacturing components, and possibly retooling of manufacturing equipment. In addition, it would be expected that the rough surface texture would also call for adjustment to the manufacturing components and processes.